1. Field of the Invention
The present invention relates to an apparatus for learn-controlling the air-fuel ratio for an automotive internal combustion engine having an electronically controlled fuel injection device which is provided with an air-fuel ratio feedback control function. More particularly, the present invention pertains to an apparatus for learn-controlling the air-fuel ratio which is capable of effectively coping with changes in the air density caused by the change in altitude or the like.
2. Description of the Related Art
An air-fuel ratio learning control apparatus such as that shown, for example, in Japanese Patent Laid-Open Nos. 60-90944 (90944/1985) and 61-190142 (190142/1986) has heretofore been adopted in internal combustion engines having an electronically-controlled fuel injection device which is provided with an air-fuel ratio feedback control function.
This type of conventional learning control apparatus is basically arranged such that a basic fuel injection quantity is calculated on the basis of parameters (e.g., an engine intake air flow rate and an engine speed), which represent an engine running condition and which are concerned with the quantity of air which is sucked into the engine, and the calculated basic fuel injection quantity is corrected by a feedback correction coefficient which is set by proportional plus integral control based on a signal delivered from an O.sub.2 sensor which is provided in the engine exhaust system, thereby calculating a fuel injection quantity, and thus effecting feedback control so that the air-fuel ratio may be coincident with a target air-fuel ratio. In an improved type of the above-described kind of conventional learning control apparatus, a deviation of the feedback correction coefficient from a reference value during the air-fuel ratio feedback control is learned for each of the predetermined engine running condition areas to determine a learning correction coefficient for each area, and when a fuel injection quantity is to be calculated, the basic fuel injection quantity is corrected by the learning correction coefficient for each area so that a base air-fuel ratio which is obtained from a fuel injection quantity calculated without correction by the feedback correction coefficient may be coincident with a target air-fuel ratio. During the air-fuel ratio feedback control, the area-wise learning correction coefficient is further corrected by the feedback correction coefficient to calculate a fuel injection quantity.
According to the above-described arrangement, when the air-fuel ratio feedback control is being effected, it is possible to eliminate the follow-up delay in the feedback control at the time of a transient engine running condition, whereas, when the air-fuel ratio feedback control is suspended, it is possible to accurately obtain a desired air-fuel ratio.
In the case where a flap type (a volume flow rate detecting type) air flowmeter is employed in a system wherein a basic fuel injection quantity Tp is determined from a throttle valve opening .alpha. and an engine speed N [e.g., a system wherein an intake air flow rate Q is obtained from .alpha. and N with reference to a map and Tp is calculated according to the equation: Tp=K.Q/N (K is a constant)] or a system wherein an intake air flow rate Q is detected by means of an air flowmeter and a basic fuel injection quantity Tp=K Q/N from the detected intake air flow rate Q and the engine speed N, a change in the air density is not reflected upon the calculated basic fuel injection quantity. However, it is possible according to the above-described learning control to cope with a change in the air density due to a change in the altitude or in the intake air temperature as long as the learning control progresses effectively.
Considering a case wherein a vehicle which is equipped with the aforementioned learning control apparatus abruptly goes up a hill, however, since a transient engine running pattern is employed while the vehicle is climbing the hill, the system in which learning control is executed for each of the engine running condition areas suffers from the problem that an area for learning cannot readily be determined; even if learning can be executed, the learning areas are undesirably limited, and learning cannot hardly progress in the greater part of the areas. Thus, when the vehicle comes into an ordinary running state, for example, at a flat area near the top of the hill, a delay is caused in the air-fuel ratio feedback control, and when the air-fuel ratio feedback control has been suspended, the base air-fuel ratio-is deviated from the target air-fuel ratio by a large margin, resulting in a failure of driveability.
The reason for the above-described disadvantages is as follows. It is necessary to correct a deviation component due to a change in the air density by learning it from the deviation of the feedback correction coefficient from a reference value during the air-fuel ratio feedback control. However, since the learned deviation also includes the deviation of the base air-fuel ratio dependent on the engine running condition which deviation is caused by variations in parts such as a fuel injection valve and a throttle body, it is impossible to separate the deviation component due to a change in the air density from the learned deviation, and it is therefore necessary to learn for each of the engine running condition areas the deviation component due to a change in the air density which must originally be able to be learned globally. Accordingly, in the case where the air density suddenly changes, for example, when the vehicle abruptly goes up a hill, learning cannot be executed for each area, so that substantially no learning control progresses.